Don't think there's much to it is there? I've never been the biggest Frank fan but it seems pretty straightforward to me.
I mean, what's not to like from their perspective? No bureaucratic BS red tape, Frank never (???) goes after the wrong person, never misses. He isn't some impossible sungod alien, he's not an ultra-rich genius with sci-fi tools and gadgets, or a mutant with impossible genetics. He's a regular soldier like them, only better at the job and he doesn't take orders from fools who shouldn't be giving them, and he's not killing random people on foreign soil who's only crime was living somewhere that's not America. Even the combat PTSD is something most cops and soldiers are intimately familiar with, they see themselves in the guy.
Granted, if you know much about Frank as a character, you know that he's not a role model of any kind, and it's arguably the cops and soldiers who should take issue with him the most. But not every dudebro in a Superman tank top knows anything about Clark, and not every woman with a Wonder Woman purse knows anything about Diana. They just know the symbol and, roughly, what it's supposed to stand for. Strength, empowerment, whatever. Frank's symbol came to represent something to these cops and soldiers, and it shouldn't have because Frank is a hot mess and a psychopath, but it's not the character they like so much as it's what they think the logo represents.